Cell and Nuclear Division (D2.1) - PDF

D 2. 1 C E L L A N D N U C L EA R D IV I S I O N  GUIDING QUESTIONS HOW CAN LARGE NUMBERS OF GENETICALLY IDENTICAL CELLS BE PRODUCED? HOW DO EUKARYOTES PRODUCE GENETICALLY VARIED CELLS THAT CAN DEVELOP INTO GAMETES? ...

D 2. 1 C E L L A N D N U C L EA R D IV I S I O N  GUIDING QUESTIONS HOW CAN LARGE NUMBERS OF GENETICALLY IDENTICAL CELLS BE PRODUCED? HOW DO EUKARYOTES PRODUCE GENETICALLY VARIED CELLS THAT CAN DEVELOP INTO GAMETES?  Compare prokaryotic CELL DIVISION cells and eukaryotic cells. PROKARYOTIC CELLS EUKARYOTIC CELLS BINARY FISSION o Mitosis – somatic body cells (46 chromosomes) o Meiosis – gametes (23 chromosomes) Compare: Give an account of similarities and differences between two (or more) items, referring to both (all) of them throughout.  CELL DIVISION IN EUKARYOTES INCLUDES INTERPHASE Interphase is the longest portion of a cell’s life and has three subdivisions: First Gap (G1) Synthesis (S) Second Gap (G2) This Photo by Unknown Author is licensed under CC BY-SA-NC This P hoto by Unknow n Autho r is licens  First Gap G G Kina se 0  CYCLINS CYCLINS ARE A GROUP OF PROTEINS THAT CONTROL CELL CYCLE PROGRESSION CYCLINS WERE DISCOVERED BY TIMOTHY HUNT, LELAND H. HARTWELL AND PAUL M NURSE, WHO FOUND THAT THE CONCENTRATION OF EACH CYCLIN WAS DIFFERENT AT EACH STAGE AND CHANGED IN A CYCLICAL FASHION. THEY WON A NOBEL PRIZE IN 2001 FOR THEIR CONTRIBUTION TO PHYSIOLOGY AND MEDICINE. CYCLINS BIND TO CYCLIN-DEPENDENT KINASES (CDKS) WHICH ARE ENZYMES CYCLINS MOVE CELLS THROUGH THE PHASES OF THE CELL CYCLE AND STOP PROGRESS AT CHECKPOINTS SOME CELLS PAUSE DURING G1 AND ENTER G0 This Photo by Unknown Author is licensed under CC BY A cell can stay in G1 indefinitely. Two factors determine the length of this pause, the type of tissue that the cell is in and the Temperature function of the Light cell. pH levels Nutrient This pause is often availability referred to as G0, or resting phase. Nerve and heart muscle cells, once CHECKPOINTS Synthesis S Kina se Checkp Second Gap Casc oint ade G G 0  The name ‘interphase’ implies a period of time between stages and that nothing much is going on. This is not the case! Interphase is when the cell is going about it’s day-to-day ‘business’ i.e. carrying out it’s programmed functions and growing (metabolic reactions, protein synthesis, DNA replication, increase in the number of mitochondria and chloroplasts) QUESTION: Imagine if cells did not have a growth phase. What would happen to the size of daughter cells with progressive rounds of mitosis? Interphase is when all of the cell’s genetic instructions are processed. Any time new cells are required, mitosis is required: Growth: An 18 year old has many more cells than an 18 month old! This P hoto by Embryonic development: You start as a zygote, one cell! Unknow n Tissue repair: Burnt, bashed, cut or eaten; dead or lost cells need Autho r is licen to be replaced. sed under CC BY- Asexual reproduction: In organisms Sea stars: These eukaryotes only reproduce by fission. SA-NC Asexual Hydra: They reproduce by budding. rep. in Yeast: These organisms reproduce by budding. animal Paramecium: They produce offspring by binary fission. s Stick insects: They use the mode of parthenogenesis sometimes to reproduce.  “MITOSIS, HOW CELLS R ENEW!” s old s is i DN) o 2 Mit (SNC s new Mitosis: The cell divides its chromosomes into 2 identical sets 4 Main stages ^ C CHECKPOINT Cytoki nesis (pinch ing) Proph ase Metap hase This Photo by Unknown Author is licensed under CC BY Anapha Pe a MA T Peo ple Mee t And This Photo by Unknown Author is licensed under CC BY Don’t forget about Cytokinesis!  Inward pinching of plasma Recall : The fluid membrane nature of the double membrane facilitates the process of cytokinesis Midway between two poles and relatively rigid 🡪 move outward from central region The cell plate is formed from the fusion of vesicles containing cell wall materials  Stage Events Gap 1 (G1) Protein Synthesis Organelles produced Cytoplasm increases in size Checkpoint 🡪 cascade 🡪 G0 Synthesis (S) DNA is duplicated Gap 2 (G2) Organelles produced Cytoplasm increases in size Checkpoint r y m m a Mitosis The cell divides it’s chromosomes into two s u identical sets I n 4 Stages Prophase (early/late) Metaphase (pre-metaphase) 🡪 checkpoint Anaphase Telophase Cytokinesis The parent cell divides into two daughter cells  A NOTE ON CANCER A DISEASE THAT OCCURS WHEN A CELL’S CYCLE IS OUT OF CONTROL CHECKPOINTS ARE MECHANISMS THAT REGULATE PROGRESSION THROUGH THE CELL CYCLE INSURING THAT EACH STEP TAKES PLACE ONLY ONCE AND IN THE RIGHT SEQUENCE. MUTATIONS OF CHECKPOINT PROTEINS ARE FREQUENT IN ALL TYPES OF CANCER AS DEFECTS IN CELL CYCLE CONTROL CAN LEAD TO GENETIC INSTABILITY. THE G1/S TRANSITION IS WHERE MOST CANCER‐RELATED DEFECTS OCCUR. MASS OF ABNORMAL CELLS ARE REFERRED TO AS A TUMOR PRIMARY TUMOR IS ONE THAT OCCURS AT THE ORIGINAL SITE OF THE CANCER SECONDARY TUMOR IS A METASTASIS THAT HAS SPREAD FROM THE ORIGINAL LOCATION TO ANOTHER PART OF THE ORGANISM FIRST YEAR 2 SKETCHBOOK ENTRY DRAW THE PHASES OF MITOSIS AND ANNOTATE THE MAJOR EVENTS. This Photo by Unknown Author is licensed under CC BY-SA  Now for Meiosis Meiosis is a reduction division... ⚫ Cell division process in which the number of chromosomes is cut in half. ⚫ Results in the formation of gametes (such as eggs and sperm) ⚫ Gametes have ½ the chromosomes of regular cells (HAPLOID = n) ⚫ Fusion of an egg and sperm results in a zygote (Somatic) ⚫ Proper chromosome number has been restored (DIPLOID = 2n) ⚫ The halving of the chromosome number allows a sexual life cycle with fusion of gametes ⚫ In a diploid human cell ⚫ 46 chromosomes grouped into 23 pairs of chromosomes (homologous chromosomes) ⚫ We use the letter n to denote the number of unique chromosomes in an organism. ⚫ In eukaryotes, there are n pairs of chromosomes. With two of each, that makes a total of 2n per cell. ⚫ Haploid = n = egg/sperm = gamete = 23 chromosomes ⚫ Diploid = 2n = zygote = somatic = 46 chromosomes  Homologous Chromosomes Homologous chromosomes are chromosome pairs of the same length, centromere position, and staining pattern with genes for the same characteristics. One homologous chromosome is inherited from the organism's mother, the other from the organism's father (23 pairs in humans = 46)  Process of Meiosis Meiosis includes two rounds of division: Meiosis I & Meiosis II. Meiosis I Prophase I, Metaphase I, Anaphase I and Telophase I Homologous chromosomes are paired While paired, they cross over and exchange genetic information (DNA) Homologous pairs are then separated, and two daughter cells are produced Meiosis II Prophase II, Metaphase II, Anaphase II and Telophase II The same as mitosis Sister chromatids of each chromosome separate Result is four haploid daughter cells  Homologous chromosomes Sister chromatids separate... separate... One diploid nucleus Four haploid nuclei One diploid nucleus First Division halves the chromosome number! Four haploid Meiosis I: Prophase I: Nuclear envelope dissolves Spindle of microtubules starts to form Homologues become closely associated in synapsis Pairing of homologous chromosomes Crossing over may occur between non-sister chromatids (of homologous pairs) Chiasmata: evidence of exchange between chromatids This micrograph shows a pair of homologous chromosomes, each with two chromatids, during prophase I of meiosis in a salamander Two chiasmata are visible (bivalent)  Crossing over can occur on any part of a chromosome The size of the section swapped between chromosomes can be almost any size The number of chiasmata on each chromatid can vary These three points alone lead to innumerable possibilities Condensation will then follow Chiasmata and Crossing over This image shows that multiple chiasmata can form within one tetrad. Genetic Variation Meiosis I Metaphase I: Spindle fibres ▪ Microtubules from opposite poles attach to each homologue, not each sister chromatid ▪ Spindle microtubules move homologous pairs to equator of the cell ▪ The orientation of each pair of homologues (maternal and paternal) on either side of equator is random and independent of other homologous pairs Genetic Variation Genetic Variation Haploid cells (between non-sister homologous chromatids...)  Mendel’s Law of Independent Assortment Mendel's law of independent assortment, states that allele pairs separate independently during the formation of gametes. This means that traits are transmitted to offspring independently of one another. Mendel’s Law of Independent Assortment and Meiosis Thomas Hunt Thomas Hunt Morgan and Meiosis Meiosis I Anaphase I: ▪ Microtubules of the spindle shorten ▪ Homologues are separated from each other ▪ Sister chromatids remain attached to each other at their centromeres Meiosis Telophase I ⚫ Nuclear envelopes form around each set of chromosomes ⚫ Each new nucleus is now haploid ⚫ Sister chromatids are no longer identical because of crossing over ⚫ Cytokinesis occurs  Homologous chromosomes Sister chromatids separate... separate... One diploid nucleus Four haploid nuclei One diploid nucleus First Division halves the chromosome number! Four haploid Meiosis II Prophase II ⚫ Chromosomes, which still consist of two chromatids, condense and become visible ⚫ The new spindle microtubules develop at right angles to the old spindle ⚫ Nuclear envelope breaks down Meiosis II Metaphase II ⚫ Chromosomes line up along equator ⚫ Each chromosome attaches to a spindle fibre by means of its centromere  Microtub ule fibres Meiosis Anaphase II ⚫ Centromeres separate and chromatids are moved to the opposite poles Meiosis II Telophase II ⚫ Chromatids reach opposite poles ⚫ Nuclear envelops forms ⚫ Cytokinesis occurs  Homologous chromosomes Sister chromatids separate... separate... One diploid nucleus Four haploid nuclei LET’S PAUSE FOR GLENN…. HTTPS://GIZMOS.EXPLORELEARNING.COM/  One diploid nucleus m e! k Ti b oo First Division tch S k e halves the chromosome number! Four haploid  Non-Disjunction Non-disjunction occurs when chromosomes don’t separate properly during meiosis (egg/sperm cell chromosomes vary) Incorrect number of chromosomes Problems with meiotic spindle cause errors in daughter cells Homologous chromosomes do not separate properly during Meiosis I (eg. stick together) Sister chromatids fail to separate during Meiosis II Too many or too few chromosomes Non-Disjunction  TRISOMY 21: 3 COPIES OF CHROMOSOME 21 (1 IN 800 BIRTHS) Trisomy 21 MALFORMATION OF THE DIGESTIVE SYSTEM DIFFERING DEGREES OF LEARNING DIFFICULTIES PHYSICAL ABNORMALITIES CHROMOSOME 21 IS THE SMALLEST HUMAN CHROMOSOME FREQUENCY OF DOWN SYNDROME CORRELATES WITH THE AGE OF THE MOTHER Down Syndrome Karyotype WWW.EXPLORELEA RNING.COM  Errors in Chromosome Division FERTILIZATION RESULTING IN 1 OR MORE EXTRA CHROMOSOMES IS CALLED POLYSOMY Errors during chromosome division Trisomy 13 – Patau syndrome Trisomy 18 – Edwards syndrome Trisomy 21 – Down syndrome RECALL: Non-Disjunction Comparison of Mitosis and Meiosis